Tfi 490 
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Copy 1 



DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 103 

TYPICAL CASES OF THE DETERIORATION OF 

MUNTZ METAL (60:40 BRASS) BY 

SELECTIVE CORROSION 

BY H^St 

HENRY S. RAWDON, Associate Physicist 

Bureau of Standards :■■- '-\ 



ISSUED DECEMBER 15, 1917 




PRICE, 10 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 
Washington, D. C. 

WASHINGTON 
GOVERNMENT PRINTING OFFICE 

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DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 103 

typical cases of the deterioration of 

muntz metal (60:40 brass) by 

selective corrosion 

BY 

HENRY S. RAWDON, Associate Physicist 
Bureau of Standards 



ISSUED DECEMBER 15. 1917 




PRICE, 10 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 
Washington, D. C. 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1917 







Fig. i. — Types of corroded Muntz metal 

(a) Surface of a badly corroded Muntz metal bolt. 

(b) Cross and longitudinal sections of the corroded -portion of bolt a, unetched (X2). The outer or attacked 

portion is copper-red; the center has the yellow color of uncorroded brass. 

(c) Boat sheathing after dezincification (Xi). The portion at the right was broken into the fragments 

shown by the fingers alone. 



0. of &. 
MN r 1918 



/8~±Ln*H 



TYPICAL CASES OF THE DETERIORATION OF MUNTZ 
METAL (60:40 BRASS) BY SELECTIVE CORROSION 



By Henry S. Rawdon 



CONTENTS 

Page 
I. Introduction , 

II. Characteristic appearance of typical cases 4 

1. Wrought bolts 4 

2 . Sheathing IO 

3 . Condenser tubes 10 

4. Corrosion of parts while under stress 10 

III. Microstructural features 13 

1. Unchanged Muntz metal 13 

2. Microstructure after deterioration 14 

3. Behavior of a brass under similar conditions 15 

4. Progress of attack within the crystals 17 

IV. Conditions influencing dezincification 17 

1. Structural composition 17 

2. Contact with electronegative substances 19 

3. Accelerating effect of the products of corrosion 19 

4. Effect of heat treatment 22 

5. Temperature 23 

6. Stresses 24 

7. Action of "flux" upon Muntz metal 26 

V. Summary 27 

I. INTRODUCTION 

Brass of the type 60 per cent copper and 40 per cent zinc, 
known under various trade names, primarily, however, as Muntz 
metal, has a variety of industrial uses, such as sheathing for boat 
bottoms; tubes for condensers; wrought forms, as bolts and rods; 
irregular extruded shapes, such as handrails for stairway balus- 
trades, etc. One very common type of deterioration of metal 
of this composition, particularly when exposed to some electrolyte 
(e. g., sea water), is selective corrosion or "dezincification," the 
term " selective corrosion " being used to signify a corrosive attack 
of certain of the microstructural constituents of the alloy rather 
than a general uniform action upon the metal as a whole. Though 
this type of deterioration of brass has been known for years and 
numerous references on this subject have appeared in the technical 

3 



4 Technologic Papers of the Bureau of Standards 

literature * the description of the various forms in which it may 
occur and of the changes produced in the metal by which it may 
be detected are very meager. The numerous samples illustrative 
of this type of metal failure submitted to this Bureau for exami- 
nation, together with the inquiries received on this subject, 
suggested the utility of a description of typical cases of metal 
affected by this type of nonferrous corrosion as an aid in the 
detection and identification of similar cases of this type of deterio- 
ration of metals. A study of the various forms in which this 
type of corrosion may occur, together with the resulting structural 
changes within the metal, also aids in defining the conditions 
which are most favorable for such deterioration to occur. 

II. CHARACTERISTIC APPEARANCE OF TYPICAL CASES 

In general, by the attack, or process of dezincification, the 
clear yellow Muntz metal having an ultimate strength in ten- 
sion of approximately 40 000 to 60 000 pounds per square inch, 
depending on the physical condition of the metal, is converted 
into a reddish-colored mass resembling poor copper, often so 
weak that it may be easily broken by the fingers alone or chipped 
and cut with a knife, though it still retains the initial shape and 
size. The red color of the attacked metal is intensified by rub- 
bing the part with a cotton swab soaked in dilute ammonium 
hydroxide and then rinsing with water. The surface is freed 
from its film of oxide and the unattacked portions assume the 
original yellow color, the corroded spots become a brownish-red. 
Below are described five typical cases selected from material of 
this kind which was submitted to this Bureau for examination. 

1. WROUGHT BOLTS 

The Muntz-metal bolt illustrated in Figs. 1 and 2 was taken 
from the keel of a seagoing lifeboat after six years' service. The 
sample shown is illustrative of the condition of all the bolts used in 
this particular case to attach the keel to the keelson. To the 
casual observer the broken bolt appears to consist of a brass 
center with a heavy deposit of porous copper covering it. The 
location of that part of the bolt which was most severely attacked 
and its relation to the position within the keelson is of interest and 
is suggestive of one of the possible contributing causes to this 
type of corrosion. This is shown in Fig. 3. 

1 See, for instance, Second and Third Reports of the Corrosion Committee of the Institute of Metals, Jour. 
Inst, of Metals, 1913, X, and 1916, XV, respectively; J. O. Arnold, Engineer, 85, p. 363, 1898; and Milton 
and Larke, Proc. Inst. Civ. Engr., 154, p. 138, 1903. 



Deterioration of Muntz Metal 




a 




Fig. 2. — Microstructural changes induced in Muntz metal by dezincification 

(o) Unattacked Muntz metal (Fig. i, b). The light-colored matrix is the zinc-rich or /3 constituent, in 
which are embedded the finger-like crystals of the a or constituent richer in copper. (X500.) 

(6) Corroded Muntz metal. At the edge of the corroded portion (Fig. 1, b) the constituent has been 
converted into a porous mass of copper (black in the figure) which still retains its former shape. ( X 500.) 

Etching in both cases, ammonium hydroxide and hydrogen peroxide. 



6 Technologic Papers of the Bureau of Standards 

It will be noted that the portion of the bolt which was most 
deeply corroded is that part which was subject to the greatest 
service stresses (transverse bending). Although apparently this 
material gave six years' service, it should be borne in mind in 




Fig. 3. — Arrangement of the Muntz metal bolts (type 1) in the keel of the boat 

The part which was most severely corroded corresponds to the portion most highly stressed (i. e., trans- 
versely) in service. 

comparing this case with the others described that the bolts were 
not constantly bathed in sea water for this period and that the 
periods of attack were intermittent, as illustrated by the con- 
centric lines in Fig. i&. 



Deterioration of Muntz Metal 




a 




Fig. 4. — Corroded Muntz metal sheathing 

(a) Before attack; the light-colored 18 matrix fills in between the a crystals. 

(6) After 7 years' exposure to sea water (see Fig. 1, c) the P constituent has been entirely changed; the a, 

apparently not at all. The numerous twinned crystals in the a is indicative that the metal was 

annealed before use. 
Magnification in both cases. (X500.) Etching, ammonium hydroxide, and hydrogen peroxide. 

7265°— 17 2 



Technologic Papers of the Bureau of Standards 



- ■■■■■■■■ — — ^M^M 




a 



b 




Fig. 5. — Corroded Muntz metal condenser tube 

(a) Appearance of the corroded tube. The darker portions are copper-red and very brittle. The metal 

here is easily cut with a knife. ( X 1 . ) 
(fc) Cross sections of parts of a. In the dark portions the metal has been corroded throughout the entire 

cross section. (X2.) Etching, concentrated ammonium hydroxide, and hydrogen peroxide, 
(c) Microstructure of the metal of a. Both a and /3 constituents are attacked; a definite zone in which 

the /3, only, is attacked separates the unchanged metal from the completely corroded portion. The 

a shows little or no evidence of annealing. (X500.) Etching, concentrated ammonium hydroxide 

and hydrogen peroxide. 



Deterioration of Muntz Metal 




a 



"W^? 



■ - 



V" 's V ^t^^llr' 





FlG. 6. — Corroded Muntz metal condenser tube 

(a) Appearance of the corroded tube. No evidence of corrosion shows on the exterior, a thin unattacked 

layer of a completely masks the corroded portion below. This outer layer is easily stripped off as 

shown. 
(6) Material of a (X250). Etching, concentrated ammonium hydroxide, and ammonium persulphate. 

The outer thin layer of unattacked a is due to some zinc being volatilized from the surface during 

some stage in the manufacturing process. 



IO 



Technologic Papers of the Bureau of Standards 




2. SHEATHING 

The sheathing shown in Figs, i and 4 was used for seven years 
upon an anchored lightship. The attack was local, certain isolated 
sheets only over the bottom of the boat were in the condition illus- 
trated; others, though much thinner than when rolled, apparently 
were in the same structural condition as when placed in position. 

3. CONDENSER TUBES 
The condenser tubes are illustrative of a case in which this type 
of corrosion is very prevalent. Both the samples shown (Figs. 
5 and 6) are from condensers using salt water. 
Type 36 (Table 1) is of particular interest in that 
it illustrates how this type of attack may be en- 
tirely hidden from view. 

The outer layer of the tube shows no evidence 
of attack, and subsequent examination showed 
that this outer portion contained but one con- 
stituent — the a. During some phase of the 
manufacturing process, the tube was evidently 
heated strongly enough so that a sufficient 
amount of zinc was volatilized from the surf ace 
as to leave the composition here such that only 
the a. constituent was present. 

The corrosive attack which proceeded outward 
from the inner surface, which was in contact with 
the sea water, proceeded up to this outer a layer; 
the layer itself, however, remained unattacked 
Fig. 7 — Diagram of and the tube appeared to be unchanged. This 
the fracture ofMuntz ou -(- er a layer can be easily stripped off from the 

metal bolthead (type , ., , , -, ,, < . -r-v- ^ 

4, Table 1) corroded metal beneath, as shown m Fig. 6. 

4. CORROSION OF PARTS WHILE UNDER STRESS 

Type 4 (Table 1 ) is particularly instructive in suggesting service 
conditions which may aid in this type of corrosion. Fig. 8 shows 
the face of the bolthead which was fractured as shown in Fig. 7. 
The head detached itself apparently spontaneously in service, 
suggesting in this respect the " season cracking " of brass. Dezinc- 
ification of the surface metal in the outer marginal area, x — x', 
occurred and penetrated to a depth of from 0.05 to 0.36 mm; the 
central area shows no corrosive attack, the fracture here having 
the appearance of a direct tension break. The most severe attack 
was near the apex of the right angle beneath the head. The easiest 
and most logical explanation is that the failure in this case is one 
of corrosion under stress. 2 The starting point perhaps may have 



2 For description of such cases of corrosion of brass under stress, see E. Jonson, Proc. Am. Soc. Test Mtls. 
p. 101, 1915; and P. D. Merica, B. S. Tech. Paper No. 83. 



Deterioration of Muntz Metal 



ii 




Fig. 8. — Muntz metal bolt corroded while under tensional stress 

(a) Face of the fractured head. The central portion x-x, has the appearance of a simple tension break; 

the marginal area x-x' which resembles a "detailed" fracture shows dezincification of the /3. (Xi.) 
(6) Edge of the fracture of area x-x. This portion shows no change in structure. X 250. 
(c) Edge of the fracture of area x-x.' The p constituent at the extreme edge has been dezincified. X250. 
Etching in both cases, ammonium hydroxide and ammonium persulphate. 



12 



Technologic Papers of the Bureau of Standards 



been an initial flaw in the material, such as a forging eraek. The 
P constituent appears to have become dezincified in the region of 
greatest stress— that is, in the sharp right angle— and to have 
preceded and accompanied the gradual or detail fracturing of this 
portion of the break until the central portion became so small that 
it broke in direct tension under the stress of the applied load the 
bolt carried. 

Below are summarized the different types described, the service 
to which the material has been subjected, the macroscopic charac- 
teristics, the changes in the microstructure, as well as the chemical 
composition where available. 

TABLE 1 
Summary of Materials Described 



Material 



Service 



1. Colt of Muntz 
metal <* 



2. Muntz metal 
sheathing.b 



3a. 



Muntz 
tube.c 



metal 



Attachment of 
bronze keel to 
oak keelson of 
seagoing life- 
boat. Six years' 
inter m i 1 1 e n t 
service. All the 
bolts showed the 
same appear- 
ance. 

Used for seven 
years as sheath- 
ing upon an an- 
chored lightship 
in latitude of 
New York (ap- 
pro*.). 



In a condenser 
using sea water 
six months. 



Macroscopic appear- 
ance of corroded 
material 



36. 



Muntz 
tube. 6 



metal 



4. Muntz metal 
bolthead.f 



In a condenser 
using sea water; 
term of service 
unknown. 



Term of service, 
two years. The 
bolt which was 
in tension was 
exposed to sea 
water. The 
head dropped off 
of its own ac- 
cord. 



In cross section it 
shows a yellow 
center and red 
outer layer and 
has the superficial 
appearance of a 
brass rod heavily 
copper coated; the 
coating is very brit- 
tle. 

Isolated sheets only 
were attacked, not 
the whole bottom. 
Sheets are some- 
what thinner at 
first and are cov- 
ered with basic 
deposits. Metal is 
brown and spongy 
and very brittle. 

Tube has a mottled 
appearance of yel- 
low and red, red 
being most abun- 
dant on inner side. 
Tube is cracked 
along one side 
where the red 
had penetrated to 
the exterior. 

The exterior of the 
tube is bright yel- 
low, and shows no 
corrosion. This 
yellow layer which 
is very thin can 
be striped off, and 
reveals the- porous 
red metal be- 
neath. 



The center of the 
fractured face has 
the appearance of 
a tension break; 
the outer margin, 
that of a detailed 
fracture. 



Microscopic struc- 
ture 



Both a and /3 con- 
stituents are at- 
tacked in the 
greater portion of 
the corroded part; 
on the inner mar- 
gin only the /S or 
zinc -rich matrix is 
replaced by "cop- 
per." 

6 constituent alone 
has been replaced 
throughout by por- 
ous "copper"; the 
a is unattacked. 



Both a and are 
attacked in com- 
pletely corroded 
parts. In outer 
margins of corro- 
sion spots only /3 
is attacked. 



Both a and p con- 
stituents have 
been attacked on 
the side of the tube 
showing greatest 
corrosion. A clear 
yellow unattacked 
layer on the ex- 
terior consists of 
the a. constituent 
only. The extent 
of the attack has 
been limited by 
this. 

The constituent has 
been attacked in 
the outer marginal 
area of the face of 
the fracture to an 
average depth of 
0.05 to 0.36 mm.; 
neither a nor (3 
havebeenchanged 
in the central area. 



Chemical com- 
position (in 
per cent) 



Initial: Cu, 59.7; 
Pb, 0.19; Sn, not 
detected; Zn 
(dif.), 40.11. 

Attacked portion: 
Cu, 93.4; Pb, 
0.06; Sn, trace; 
Zn, and basic 
chlorides, etc. 
(dif.), 6.5. 

Initial (according 
to specifications) : 
Cu, 59 to 62; Zn, 
41 to 38; Pb 
(max.), 0.60. 

Final: Cu, 75.79; 
Pb, 0.68; Sn, 
0.36; Zn, 20.01; 
Fe, 0.16; S, 0.07; 
CI, trace. 

Initial: Cu, 61.7; 
Pb, 0.2; Fe, 
trace; Sn, not de- 
tected; Zn (dif.), 
38.1. 



Copper, 60.95; lead, 
0.13; tin, 0.59; 
Fe, 0.00; Zn(dif.), 
38.33. 



a See Figs i and 2 a and b. 
b See Figs, ic and 4 a and 6. 



cSee Fig. 5 n, b, and , 
d See Fig. 6 a and 6. 



'See Fig. 7 and 8 a, b, and , 



Deterioration of Muntz Metal 



13 



III. MICROSTRUCTURAL FEATURES 
1. UNCHANGED MUNTZ METAL 

In order to understand clearly the method by which such pro- 
nounced changes in structure, composition, and properties are 
brought about, a knowledge of the microstructure of the alloy is 

II00°C 



I000°C 



900 o C 



eoo°c 



70CC 



600°C 



SOQ'C 



4-0O°C 

















S. a+L 


>\ 


Li 


auid 












^^2 


f^^v* 


/ 




a 






v /3 


/ 








la. 


A 1 


■/3+7* 




























■4ra'C 























70 



60 



50 



100 9o eo 

Percentage, of Co/Ofi&f. 

FlG. 9. — Structural diagram of the copper-zinc alloys 



40°/. 



essential. The metal has a duplex structure, as is shown by Fig. 
2a. As an example, consider the alloy of 60 per cent copper and 
40 per cent zinc, the crystals or "grains" which, when heated 
sufficiently, are homogeneous throughout, on cooling break up into 
a complex of two constituents, as shown by the structural diagram 
(Fig. 9). This is brought about by a crystallization within and 



14 Technologic Papers of the Bureau of Standards 

throughout the previously homogeneous grain by which the crys- 
tallites or "fingers" of the new constituent form out of the sub- 
stance of the mother matrix and remain embedded in it. The 
ground mass (Fig. 2a) is the zinc rich or /3 3 constituent ; the darker 
fingers embedded in this matrix, the a constituent, which in its 
saturated condition consists of very nearly 64 per cent copper 
and 36 per cent zinc. The percentage composition of the zinc- 
rich or jS matrix will vary with the ultimate composition of the 
alloy in question, but it is always considerably richer in zinc than 
is the a phase. 

In passing, it should be remarked that the color of the two 
constituents may be varied considerably by the etching medium 
used. However, with slight etching by means of alkaline reagents, 
the /3 always appears yellow at first; deeper etching or other con- 
ditions — e. g., slight oxidation of the surface- — may cause it to 
appear darker than the a. In no case, however, was the j3 found 
to appear red as is stated by some observers 4 of this alloy. 

2. MICROSTRUCTURE AFTER DETERIORATION 

There is to be observed a very sharp line of demarcation between 
the attacked and unattacked portions. The change in compo- 
sition and properties is not a gradual one, but is very abrupt and 
complete. After corrosion the attacked portion of the metal con- 
sists of a porous mass, apparently of copper, still showing indis- 
tinctly its initial structural arrangement. Often well-defined lami- 
nations parallel to the corroded surface are seen, corresponding 
undoubtedly to the intermittent periods of service to which the 
metal was put (Fig. 16). The interstices of the porous "copper" 
are filled with basic chloride deposits — the products of corrosion. 
These are very abundant in the inner layer adjacent to the un- 
attacked metal and under the microscope appear blue-gray in con- 
trast to the red of the copper. 

At the boundary surface between the changed and the un- 
changed portions is plainly seen the method of attack. Figs. 2b 
and 5c show this. The /3 or zinc-rich matrix is the constituent 
which is first acted upon, and appears to be replaced by one higher 
in copper; in reality it is not a "replacement." The preferential 
attack of the /? phase is to be attributed to its higher zinc con- 
tent, which renders it more electropositive than is the a constitu- 

3 This assumption disregards the question whether this constituent is /3', a polymorphic modification, 
or an intimate mixture of two other phases, a and y. (See Jour. Inst. Metals, 1912, VII, p. 70, and 1912, 
VIII, p. si.) 

4 Shepherd, Jour. Phys. Chem., 1904, VIII, p. 421; and Gulliver, Metallic Alloys, p. 269. 



Deterioration of Muntz Metal 15 

ent which is richer in copper. When an electrolyte is present 
and electrolysis occurs either by means of the application of an 
external emf, by contact with some metal which is less electro- 
positive than either constituent of the alloy or by reason of the 
electrochemical difference between the a and /3 phases, the (3 is 
always attacked first; its zinc largely passes into the solution 
leaving behind the porous copper masses occupying the spaces 
initially filled with the jS matrix. This type of corrosion may be 
then regarded as an electrolytic "leaching out" of the zinc. The 
a constituent is later often attacked, so that the copper content 
of the whole mass may be raised as high as is indicated in Table 1 . 

A definite, though narrow, intermediate zone in which the /3 is 
completely attacked always precedes and separates the layer in 
which the attack upon the a begins from the unchanged material. 
It is to be expected from theoretical considerations that the rate 
of corrosion of pure j8 or of pure a brass is much less than that of 
the a-/3 metal. Desch and Whyte 5 have shown that such is the 
case. 

The extent to which the a constituent is affected determines 
largely the properties of the corroded metal. In the case of the 
Muntz-metal sheathing described, the a remained unattacked 
even after seven years of exposure to sea water. The sheet, when 
cleaned, still retains much of its yellow color and is not nearly so 
brittle as some of the other samples. The metal after the attack 
consists essentially of a skeleton of a, the pores and interstices of 
which are filled by the pulverulent copper which resulted from the 
corroded /? crystals. In the bolts described (type 1), the attack, 
though similar, is very different in regard to the rate at which the 
corrosion of the a phase follows that of the /3. The attack of this 
constituent follows almost immediately that of the /? grains, the 
intermediate layer in which only the /3 is attacked averaging only 
0.25 mm in depth. The same is true of the condenser tube 
(type 3a) ; the deterioration of the a follows very quickly that 
of the (3. 

3. BEHAVIOR OF a BRASS UNDER SIMH.AR CONDITIONS 

The behavior of a brass — that is, brass in which the copper is 
high enough so that the /3 constituent is entirely absent — -when 
placed in comparable conditions is of interest. Fig. 106 illus- 
trates how this behavior differs from that of metal containing 
both phases. The material here shown is from brass bolts which 

6 Jour. Inst, of Metals, 1914, XI, p. 235; ibid., 1915, XIII, p. 80. 



i6 



Technologic Papers of the Bureau of Standards 




a 




Fig. io. — Method of dezincification 

(a) Method of attack within the fi crystals. The attack here as shown by the large central grain is not 
a gradual one but abrupt and complete. (X500.) Etching, ammonium hydroxide and hydrogen 
peroxide. 

(6) Corrosive attack of a brass. The material (bolts) was used in a "water" rheostat. The attack induced 
by the external emf is very complete and shows a slightly greater rate of attack at the crystal 
boundaries. (X500.) Etching, ammonium hydroxide and hydrogen peroxide. 



Deterioration of Muntz Metal 1 7 

were used in the construction of a "water rheostat." The macro- 
scopic appearance is very similar to that of attacked Muntz metal. 
Fig. 106 shows, however, that the method of attack is very dif- 
ferent. The normal (i. e., unchanged) metal consists of the poly- 
hedral crystals of the a solution of zinc in copper with the numer- 
ous twins characteristic of annealing after strain. In any section 
the line separating the corroded portion from the unchanged 
center is very smooth and even. A few sharp bends in this line 
are to be noticed. These peaks are found to correspond to crystal 
boundaries, thus indicating a slightly faster rate of attack on the 
crystal surfaces, which is to be expected from theoretical consid- 
erations. Aside from these variations the whole "line of attack" 
is very even and shows no such preferential action as when both 
constituents are present. 

4. PROGRESS OF ATTACK WITHIN THE /3 CRYSTALS 

In Fig. 10a is shown the method of attack within the individual 
/3 crystals of Muntz metal. The advance within each /? crystal is 
very uniform and similar to that within any crystal of a when 
but one phase is present. The line separating the unchanged 
from the porous "copper" which results from the attack is very 
distinct ; the change is not a gradual one even here, but is abrupt 
and complete. 

The above is true, however, of which is in an annealed state. 
Metal which was quenched (the sample used was quenched in 
water from approximately 67 5 ° C) shows an attack of the /3 very 
different from that ordinarily observed. The /3 crystals in this 
case are attacked most readily along certain planes, often along 
two sets of intersecting planes within the same crystal (Fig. 
11 a and b). The attack then proceeds from these planes of 
attack until the whole crystal has been converted into the same 
condition as occurs more directly in the annealed material. 

IV. CONDITIONS INFLUENCING DEZINCIFICATION 

1. STRUCTURAL COMPOSITION 

Although a brass is subject to dezincification, the attack is not 
nearly so rapid as when both a and /3 constituents are present. 6 
Several of the samples of Muntz-metal sheathing, which were 
heat treated (see below) previous to subjecting them to conditions 
favorable for corrosion, were heated strongly enough so that zinc 



6 Second Report of Corrosion Committee, Jour. Inst. Metals; 1913. 



i8 



Technologic Papers of the Bureau of Standards 




a 




Fig. ii. — Method of dezincification 

The material was quenched after heating to 675° C. The /3 crystals are attacked most readily along certain 
planes arranged approximately in parallel sets in any individual crystal and resembling twinning 
planes. The material was thin sheathing and the attack was at right angles to the surface photographed. 
(X500.) Etching, concentrated ammonium hydroxide and ammonium persulphate. 



Deterioration of Muntz Metal 19 

was volatilized from portions of the surface, leaving a layer of a. 
This layer was in every case free from dezincification during the 
course of the experiment and served as a protective covering for 
the a-/3 alloy beneath. One of the samples of condenser tubing 
described illustrates the comparative freedom of a from attack. 
That the attack of the a does not follow that of the /3 at the same 
rate in all corroded samples is very evident from the types de- 
scribed. In the case of the corroded sheathing, the a is intact 
and apparently little changed even after seven years' exposure 
to sea water, while in others a few months' exposure disintegrates 
both constituents; the attack of the a very closely follows that 
of the /3 in such cases. 

2. CONTACT WITH ELECTRONEGATIVE SUBSTANCES 

If a sample of Muntz metal is in intimate contact, while im- 
mersed in an electrolyte (e. g., solution of sodium chloride), with 
a strongly electronegative metal (i. e., more electronegative than 
the a or /3 constituents), dezincification of those portions of the 
brass which is in immediate contact with the other metal will 
readily occur. As part of the study of the material of type 1, 
samples of the unchanged metal of the center were placed in con- 
tact with copper, others with steel, in a 5 per cent solution of 
sodium chloride. This electrolyte was chosen since this type of 
corrosion usually occurs in sea water. The copper readily induces 
dezincification, evidences of the attack showing after 12 hours' 
exposure, but only at or very near the point of intimate contact 
(Fig. 1 2a and b.) The attack is not general over the entire surface 
of the brass sample. No dezincification of the brass occurred in 
the case of the brass-iron couple. Although intimate coatact 
with the proper substances will induce and accelerate the corrosive 
attack of brass, in none of the types previously described can 
this condition be assigned as the principal cause of the attack. 

3. ACCELERATING EFFECT OF THE PRODUCTS OF CORROSION 

The accelerating effect of deposits of basic zinc chloride formed 
on the surface of the corroded specimen by the corrosion of the 
brass, upon the further attack of the metal has been known for 
some time and has been previously described. 7 The exact way in 
which this acceleration occurs, however, is not clear, as yet. 
The basic zinc-chloride deposits have been described by some as 

7 C. H. Desch, Physical and Mechanical Factors in Corrosion, Jour. Faraday Soc: 19T5. (See, also, 
footnote 1.) 



20 



Technologic Papers of the Bureau of Standards 





a 




d 

EiG. 12. — Illustration of various conditions affecting dezi,. ~ification 

(0-6) Samples of Muntz metal which were in contact with copper while immerse, 'n sodium chloride 
solution. In a the wire was inserted in the central hole; in b it was wrapped a_ound the sample. 
The attack is immediately adjacent to the copper wire. ( X 2) . 

(c) Muntz metal sample which has been immersed in 5 per cent sodium chloride solution. The dark 

spots are corroded areas which formed beneath adhering deposits of basic zinc chloride. (X2.) 
unetched. 

(d) Cross section of one of the spots shown in c. (X500.) Etching, ammonium hydroxide and hydrogen 

peroxide. The dark band is the basic zinc chloride deposit which accelerated the attack of the metal 
beneath. A copper layer, shown in the upper two corners, was deposited in the sample to protect 
the edge of the specimen during the polishing. 



Deterioration of Muntz Metal 



21 




a 







Fig. 13. — Illustrations of different conditions affecting dezincification 

(a) Base of V-groove in stress-corrosion specimen (No. 11, Table 2). The sample was nickel-plated and 
then copper-plated to protect the edge during polishing. The/3 has been dezhicified at the extreme 
base of the notch. (X250.) 

(6) Sample of Muntz metal dezincified by heating in molten zinc chloride. Both a and & are completely- 
attacked in the surface layer, an intermediate layer in which the ft only, has been dezincified sepa- 
rates this portion from the uncorroded part. (X250.) Etching for both a and 6, concentrated 
ammonium hydroxide and ammonium persulphate. 



22 Technologic Papers of the Bureau of Standards 

acting "catalytically," by others as being strongly electronegative 
toward both the a and /5 constituents and thus inducing dezinci- 
fication in the same way that copper does. Heavy deposits of 
basic zinc chloride and carbonate were observed on the inside of 
the condenser tubes and on the outer or exposed surface of the 
sheathing. In type i (wrought bolts used for attachment of a 
boat keel), though no pronounced deposit on the outside was 
found, the "copper" coating was observed to be impregnated 
with a similar substance. The deterioration of the metal directly 
adjacent to the deposits is in nearly every case entirely complete; 
that is, both a and /? are attacked. The degree to which the metal 
is attacked decreases with the distance from these deposits. In 
the examples described below (sees. 4 and 5), closely adhering 
hard, smooth specks of white basic zinc chloride were often found 
on the surface of the specimen when allowed to stand in the 
sodium-chloride solution. The metal immediately beneath such 
deposits was invariably in the red corroded condition. Fig. 12 
c and d illustrate the appearance of such corroded samples. 

4. EFFECT OF HEAT TREATMENT 

It has been suggested that the differences shown by different 
lots of Muntz metal in their resistance to this type of corrosion 
is to be attributed to unavoidable variations in the heat treatment 
such samples have received, 8 and that thorough annealing of 
the alloy at 650 to 8oo° C will prevent the preferential attack 
of the /3, or zinc-rich, constituent. A series of samples of new 
sheet Muntz metal of the following composition were annealed 
for periods varying from 30 minutes to 4 hours; one series at 
approximately 370 C (360-3 73 ° C), the second at approximately 
6 4 o°C (62 5 -655°C): 

Percent 

Copper 61. 35 

Lead 46 

Tin , 16 

Iron 20 

Zinc (difference) 37-83 

These two annealing temperatures were chosen so as to illus- 
trate the possible effect of heating below or above the trans- 
formation which occurs in the /? constituent at 470 C. The 
samples were then immersed in 5 per cent solutions of sodium 
chloride for a period of 70 days, each sample being suspended 

8 Second Report of Corrosion Committee, discussion by Sir Gerard Muntz, Jour. Inst. Metals, X, 1913, 
p. 101. 



Deterioration of Muntz Metal 23 

by silk thread, proper care being taken to avoid contact with 
each other or with the glass sides of the container. No effort 
was made to regulate the temperature of the solution; it 
varied between 12 C and 20 C. At the close of this period 
a microscopic examination of cross sections of the samples showed 
that each one had been attacked to a slight degree. This attack, 
which is confined to the /3 constituent, was not general over the 
entire surface but was confined to small local centers. Each 
sample showed closely adhering white specks of basic zinc chlor- 
ide; the metal under such speck showed a dezincification of the 
j8 constituent to an average maximum depth of 0.39 mm (0.0156 
inch). The depth to which this attack of the j8 constituent had 
penetrated varied from 0.31 mm (0.0124 inch) to 0.05 mm (0.0020 
inch), but apparently bears no relation to the previous heat 
treatment of the sample but rather to the character of the sur- 
face, a slightly roughened or pitted surface forming a much 
better support for holding the zinc-chloride deposits, which form 
from the slight general attack of the metal when first immersed, 
than a smoother one does. The samples heated for two and four 
hours at the higher temperature, 640 C, had lost some of the 
zinc by volatilization, so that much of the surface layer is consisted 
of but the a constituent. No attack of such portions was observed. 

Samples of the same material used above were quenched in 
water after heating to a temperature of 690 C. At this tem- 
perature both constituents, a and /?, still persist (Fig. 9). The 
attack in this case was more rapid, the /3 having been attacked 
to a depth of 0.125 mm (0.005 inch) after 13 days' immersion 
in 5 per cent sodium-chloride solution. 

The obvious conclusion to be drawn is that thorough anneal- 
ing of the metal is not sufficient to entirely inhibit the selective 
corrosion of the material. The accelerating influence of the 
adhering deposits of the basic zinc chloride, however, appears to 
be a necessary condition for such an attack of the annealed metal. 

5. TEMPERATURE 

Practical cases of the effect of temperature upon this type of 
corrosion have been brought to the attention of this Bureau. 
One striking example is that of the corrosion of the tubes of a 
condenser using sea water. The outlet pipe, in which the water 
was always considerably warmer than the incoming stream, 
corroded badly enough to require replacement several times 



24 



Technologic Papers of the Bureau of Standards 



Z 



2 



-7i, 



t7$io*? specimen 



"y-SOtftort 



— or'/ ' /ayet*. 






while the inlet pipe was still intact and in service. The effect 
of temperature upon corrosion of similar material has also been 
studied by Bengough. 9 

6. STRESSES 

The appearance of two of the types described (i and 4) suggest 
the possible accelerating influence of the stresses to which these 

samples were subjected as one of the 
causes contributing to corrosion of 
this type. To illustrate this, a series 
of specimens were subjected to ten- 
sional stress while immersed in a 5 
per cent solution of sodium chloride. 
The specimens were turned down to 
a diameter of approximately one- 
fourth inch, a very narrow V notch 
was then cut so that the diameter at 
the point of the notch was reduced 
to one-eighth inch. The purpose of 
the experiments was to see whether 
along the line of maximum stress — 
that is, the base of the V groove — the 
metal corroded more uniformly and 
readily than elsewhere on the same 
specimen . The samples were removed 
periodically from the solution and 
examined to make certain that no 
deposits of basic chloride had formed 
within the groove which might thus 
accelerate the attack at that point. 
Fig . 1 4 shows the method of the set-up . 
The material used was of the a-$ type of structure and of the 
following composition: 



4=h 



. rts66*r sto/oper 



T*- 



-fH 



Support for fvaioAt 



Fig. 14. — Arrangement for apply- 
ing tensional stress while speci- 
men was immersed in sodium 
chloride solution 

The specimen is one-eighth inch in diam- 
eter at the base of the groove. 



Copper . 
Tin 



Per cent 
. 62. 26 



Lead 00 

Iron 00 

Zinc (difference) 35. 56 

The samples were used both in the cast condition and after an- 
nealing at approximately 520 C. A determination of the mechan- 



9 Second Report of Corrosion Committee, Tour. Inst. Metals; 1913. 



Deterioration of Muntz Metal 



25 



ical properties of the material as cast and as annealed gave the 
following results: 



Sample 




Annealed 



Ultimate strength, pounds per square inch 
Proportional limit, pounds per square inch 

Reduction of area, per cent 

Elongation, (2"), per cent 

Fracture 



27 200 

7 500 

35.7 

25.0 

V) 



a Irregular; lemon-colored spots. 



b Flaw at center. 



Table 2 gives the results obtained after immersion for varying 
periods in 5 per cent sodium-chloride solution, the sample being 
under tension, which was maintained constant for the specimen 
throughout the period. The temperature was that of the room, 
which varied between 10 and 18 C (approximate). 

TABLE 2 
Corrosion of Muntz Metal While Stressed in Tension 



Specimen 


Stress 


Period 


Physical state of mate- 
rial 


Results a 


3a 6 


lbs./in. 2 

6 400 

16 000 

6 400 
9 600 

12 800 
9 600 

12 800 

9 600 


Days 

60 
17 

59 
10 

35 

60 

40 
58 




No dezincification at base of V groove. 

/3 is attacked in base of groove to a depth of 

0.25 mm. 
No dezincification in groove. 


3b 6 


do 


4 


do 


5 


Annealed; 50 minutes 
at 515° C and cooled 
in air. 

do 


No dezincification in groove. A network of 


6 .. 


very fine hairlike intercrystalline cracks 
opened up in the metal. The metal imme- 
diately adjacent to the cracks was dezinci- 
fied, both a and /3 being attacked. 


10 


do 


was attacked in base of groove to a depth of 


11.. 




0.25 mm. The a was slightly attacked. 
Dezincification of the /S to a depth of 0.16 mm 


12 


Quenched from 685° C 
in cold water. 


at the base of the groove occurred (Fig. 13a). 
No dezincification was observed in the groove. 







a The slight dezincification induced by the rubber stoppers or by small adhering specks of basic zinc 
chloride on the cylindrical surface of the specimen is not included here. Fig. 130 illustrates this attack. 

6 3a and 3b are the same specimen; after 60 days the load was increased to 16 000 pounds; that is, above 
the proportional limit. 

The tension tests made on samples of the material used as well 
as the fine cracks noted in some of the samples (some were dis- 
carded entirely for this reason) show that the material was not 
very uniform. The results of the tension tests are to be regarded 
as approximate only for the material 



26 Technologic Papers of the Bureau of Standards 

Though the results are not to be regarded as conclusive, they are 
strongly suggestive that brass of the a-f3 type highly stressed is 
more prone to selective corrosion than when not so, particularly 
when the stress is highly localized; for example, at the base of a 
fine hair crack or a narrow groove. Of the eight samples used, the 
four which showed dezincification were all stressed above the pro- 
portional limit. Both specimens 3a and 4, which were stressed 
below the proportional limit, failed to show any trace of attack 
even after 60 days' exposure. It should be noted, however, that 
all which were stressed above the proportional limit were not alike 
in developing dezincification. Specimen No. 12 (9600 pounds 
load), which was quenched, is not strictly comparable with the 
others in that the mechanical properties are not necessarily the 
same as those in the cast condition. Also, this sample lacked the 
decided duplex structure shown by the others ; this may be due to 
a slight difference of chemical composition. No adequate reason 
can be assigned for the failure of specimen No. 6, which was loaded 
above the proportional limit to develop dezincification. The 
change induced in the surface layer by the machining which the 
specimens received may be suggested as one of the possible con- 
tributing causes. The surface flow of the constituent in some 
cases is sufficient to entirely cover the /3 and thus protect it from 
the attack of the electrolyte. The results of the stress-corrosion 
tests, while not entirely conclusive, are in keeping with the general 
appearance and service behavior of some of the types described 
(1 and 4, Table 1). 

Metal which has been quenched from a relatively high tempera- 
ture may be readily shown to be internally stressed, due to the 
different rates of cooling of the outer layers relative to the interior, 
the differential rate of expansion of the different constituents, if 
more than one is present, etc. The peculiar manner in which the 
crystals of Muntz metal are corroded when the specimen has been 
quenched (Sec. III-4, above) suggests that this difference as com- 
pared with the manner of attack in the annealed metal may be 
induced by the stressed condition of such crystals resulting from 
the extremely rapid rate of cooling. 

7. ACTION OF "FLUX" ON MUNTZ METAL 

Muntz metal which has been cleaned by the use of zinc chloride, 
ammonium chloride, etc., often used as a "flux " for cleaning metal 
surfaces, by heating the metal and rubbing with a swab saturated 



Deterioration of Muntz Metal 2 7 

with a solution of the substance used often assumes a red color. 
That this reddening of the surface is a particular case of dezincifi- 
cation is shown by microscopic examination of cross sections of the 
metal taken through such reddened areas. Fig. 13& shows the 
result of heating a sample of Muntz metal in molten zinc chloride; 
in the surface layer both a and j3 are attacked, a thin intermediate 
layer in which only the jS is attacked precedes the attack upon the a. 
The suggestion is offered that metal which has been so cleaned 
and, consequently, which shows dezincification to a slight extent 
may be, perhaps, more readily attacked in service, particularly if 
the metal contains any slight surface cracks and depressions which 
might serve to hold the chloride and thus act as centers for later 
corrosion in service. 

V. SUMMARY 

1. The deterioration of Muntz metal by selective corrosion is 
illustrated by four types, including tubing, sheets, and forgings. 
The metal becomes red in color, very weak, and brittle by this 
type of corrosion, which takes place when the piece is exposed to 
some electrolyte. 

2. The metal has a duplex structure, one of the constituents, /3, 
being very much higher in zinc than the other, a. In all cases, 
the selective corrosion consists of a preferential attack of the zinc- 
rich constituent and the formation of a porous "copper" mass 
which fills the network of holes previously occupied by the 0. 
This attack may or may not be followed by that of the second 
constituent. 

3. The rate at which the a phase is attacked is variable. The 
attack of this constituent may follow almost immediately that of 
the or may be indefinitely delayed. 

4. A state of dezincification may result in a brass under certain 
conditions, although the method of attack as revealed by the 
microstructure is different. 

5. The line of demarcation between attacked and unattacked 
portions is very sharply defined. The action is not a gradual one, 
but the change is an abrupt and complete one even within the 
individual $ crystals. 

6. One of the most important conditions favorable to this type 
of corrosion, which occurs commonly while the metal is immersed 
in sea water, is the accelerating effect of the closely adhering 
deposits of basic zinc chloride resulting from the attack of the 
metal. Other influences which may accelerate the rate of attack 



28 Technologic Papers of the Bureau of Standards 

are contact with substances more electronegative than either the 
a or /? constituents, an increase of temperature, and the effect of 
stresses to which the material may be subjected while in service. 
7. Thorough annealing of Muntz metal does not entirely pre- 
vent the selective corrosion of the /3 constituent. 

Washington, June 4, 191 7. 



